It can do this by another electron colliding with it or by absorbing a photon of the exact energy.
When moving down a level the electron must lose the exact amount of energy when making the transition.
It releases this energy as a photon of energy equal to the energy it loses.
E1 is the energy of the level the electron starts at and E2 is the energy of the level the electron ends at
Excitation
When an electron gains the exact amount of energy to move up one or more energy levels
De-excitation
When an electron gives out the exact amount of energy to move back down to its original energy level
Ionisation
An electron can gain enough energy to be completely removed from the atom.
The ground state and the energy levels leading up to ionisation have negative values of energy, this is because they are compared to the ionisation level. Remember that energy must be given to the electrons to move up a level and is lost (or given out) when it moves down a level.
Line Spectra
Atoms of the same element have same energy levels. Each transition releases a photon with a set amount of energy meaning the frequency and wavelength are also set. The wavelength of light is responsible for colour it is. We can analyse the light by using a diffraction grating to separate light into the colours that makes it up, called its line spectra. Each element has its own line spectra like a barcode.
To the above right are the line spectra of Hydrogen and Helium.
We can calculate the energy difference that created the colour.
If we know the energy differences for each element we can work out which element is responsible for the light and hence deduce which elements are present.
We can see that there are 6 possible transitions in the diagram to the left, A to F.
D has an energy difference of 1.9 eV or 3.04 x 10-19 J which corresponds to a frequency of 4.59 x 1014 Hz and a wavelength of 654 nm – red.